Traditional electronic switch fabric design is typically driven by power, area, and cost constraints. Consequently, an important metric when choosing a topology for an electronic switch fabric can be the total number of switches in the fabric. Optical or photonic switch fabrics are limited primarily by the number of switches encountered in a signal path (e.g., hops). Where the total number of switching elements may still be an important determiner of cost, power, and area, the primary limiters of photonic switch scale are typically signal integrity constraints determined by the number of switch stages encountered (e.g., hops), and also by the number of other components such as waveguide crossings.
A switch-and-select topology (also known also as a tree-multiplexer switch matrix) provides strictly non-blocking routing functionality. The number of switch hops in an N×N switch-and-select fabric scales according to the order of log(N), similar to a rearrangeably non-blocking Benes topology, wherein N is the number of input ports and output ports. Whereas, a crossbar topology scales in switch hops according to O(N). Switch-and-select topology uses, for example, 1×2 and 2×1 switching elements. In a photonic switch fabric, the switching elements can be realized from a variety of different devices, including, for example, ring resonators or Mach-Zehnder interferometers, which are constructed with 2 inputs and 2 outputs. The switch-and-select disregards one input or one output of these naturally 2×2 switch elements, and this results in favorable crosstalk propagation effects.